Contr. Dudley Herb. 1: 227. 1936.
Stems 1–2 m, paniculately branched; herbage sparsely hispid, hairs simple, forked, and stellate. Leaf blades 5- or 7-lobed, 5–10 cm wide, lobes lanceolate to triangular, base truncate to cordate, margins with coarse rounded to pointed teeth. Inflorescences solitary flowers or few-flowered clusters forming open panicles; involucellar bractlets linear to linear-lanceolate, 5–10 × 1 mm, 1/3–1/2 calyx length. Flowers: calyx 15–20 mm, lobes lanceolate to ovate-lanceolate, 10–15 mm, longer than wide, exceeding tube, hirsute with few-rayed hairs 1–2 mm; petals deep rose-purple, 1.5–2.5 cm.
Phenology: Flowering Jun–Aug.
Habitat: Gravelly streamsides and open hillsides, sage brush shrub-steppe to lower Pinus ponderosa zones
Elevation: 100–1500 m
Iliamna longisepala is distinctive in its long calyx and calyx lobes. The species is rare and limited to the eastern side of the Wenatchee Mountains in the arid transition zones over a total distance of about 120 kilometers in Chelan, Douglas, and Kittitas counties.